This application claims the priority of German Patent Document 199 51 584.0, filed Oct. 27, 1999, the disclosure of which is expressly incorporated by reference herein.
Device for generating electrical energy, having a fuel cell, which is assigned additional units for starting and for operation, and method of operating the device.
The invention relates to a device for generating electrical energy, having a fuel cell, which is fed with an oxidant by a compressor, and having a high-pressure compressor intended to supply a fuel-gas production system with air. In addition, the invention relates to a method of operating a device of the above-described type.
A circuit arrangement for supplying electrical energy to a network containing a fuel cell and an accumulator circuit has been disclosed, in which the accumulator supplies the energy during the starting operation (German Patent DE 198 10 468). The accumulator circuit in this arrangement is connected via one or more DC/DC converters to the fuel-cell network, to which the drives of auxiliary units, such as a compressor for delivering the combustion agent and/or air, are also connected. At the beginning of a starting operation, the accumulator supplies the energy for the auxiliary units. After the starting operation, that is, say during rated operation, the accumulator is charged via the DC/DC converter.
A method is known for starting a fuel-cell vehicle, which is driven by an electrical drive unit which is fed by the fuel cell. The fuel-cell vehicle contains a fuel cell which is fed with a combustion agent, for example hydrogen, via a line in which a valve and a pressure regulator are arranged. The fuel cell is fed with an oxidant, for example air through another line, in which an air filter, an air-mass meter and a compressor are arranged. A starter motor and an electric motor drive the compressor. In order to start the fuel cell, the starter motor is supplied with power by a starter battery. During normal operation, the fuel cell supplies the energy needed to operate the electric motor. By means of a current controller, the is rotational speed of the electric motor, and therefore the speed of the compressor, are set in order to influence the oxidant mass flow which, in turn, influences the output from the fuel cell (German DE 43 22 767 A1).
The fuel cell is supplied with the oxidant, for example air, by a compressor both during the starting phase and during normal operation. The fuel gas needed for the operation of the fuel cell, in particular hydrogen, is often produced from hydrocarbons. The fuel-gas production system needs air, which is fed by a high-pressure compressor into the fuel-gas production system in a manner known per se. Either DC motors or AC motors or three-phase motors need to be used for the compressor drives. AC motors or three-phase motors have to be connected to the DC network of the fuel cell via inverters. During the starting phase of the fuel cell, a starter battery supplies the auxiliary or additional units, such as a compressor or a high-pressure compressor, with energy. In order to supply the fuel cell with cooling water which, although it is not needed to start the fuel cell, is needed during operation under relatively high loads, a pump is necessary, and is driven, for example, by an AC or three-phase motor connected to the electrical outputs of the fuel cell via an inverter.
With the auxiliary units, such as pumps and compressors, driven by individual motors, possibly via inverters, the result is a relatively complicated arrangement with a corresponding large space requirement and a relatively high weight. As a result of this arrangement, the overall efficiency of the fuel-cell system is also considerable reduced.
The invention is based on solving the problem of further developing a device for generating electrical energy, having a fuel cell, which is assigned additional units for starting and for operation, to the extent that, by means of a combination of the drives needed for the additional units, the device is simplified and the overall efficiency of the device is improved. In addition, the object of the invention is to specify a method of operating such a device.
According to the invention the problem is solved by a unit having a first and a second synchronous machine, with the two synchronous machines being coupled rigidly to each other as a unit and connected via an inverter to a storage battery, and for the second machine to be connected via an inverter to the electrical outputs of the fuel cell, by the unit being mechanically rigidly coupled at least to the high-pressure compressor and the compressor and by the inverters and the field windings of the synchronous machines being connected to a control unit which, during starting operation, controls the first synchronous machine as a motor and the second machine at idle and, after the end of the starting operation, controls the second machine as a motor and the first as a generator. With the device according to the invention, individual motor drives are dispensed with. The first synchronous machine operates as a generator after the end of the starting phase of the fuel cell and charges the storage battery and supplies loads which are connected to the storage battery. The starting phase or the starting operation is concluded when the fuel cell is ready to operate and outputs the appropriate operating voltage. The loads can be motors, heating resistances and so on. The circuits connected to the fuel cell are electrically isolated from the circuits connected to the storage battery, so that a DC/DC converter with a large step-up ratio for the different voltages of the fuel-cell network and the storage battery network is rendered superfluous. In the starting phase, no load is connected to the electrical outputs of the fuel cell, so that the output fed from the storage battery to the synchronous motor, to operate the compressor and the high-pressure compressor, is adequate to bring the fuel cell into the operating state desired for load operation. To supply the fuel-gas device with air, it is sufficient for the high-pressure compressor to have an output which is smaller by about one power of ten than the compressor.
In an expedient embodiment, the high-pressure compressor is connected to the unit via a step-down gear mechanism, in order to permit the motors driven by the same unit to operate in the most favourable rotational speed range.
In a preferred embodiment, the shaft of the high-pressure compressor is rigidly coupled to a water pump and a bypass equipped with a controllable valve is connected in parallel with the water pump and the water pump is connected by an output line to the cooling device of the fuel cell, whose cooling-water feed can be shut off by a further valve.
The water pump for cooling the fuel cell is driven by the same unit as the compressors, which eliminates the need for a separate motor. During the starting of the fuel cell, the cooling-water feed to the fuel cell is shut off by a valve and the bypass is switched through.
It is expedient if the field windings of the synchronous machines are each connected to the storage battery via solid-state switches. The field windings are therefore fed via the network connected to the storage battery.
The first synchronous machine preferably has about 10 to 20% of the output of the second synchronous machine. This output is adequate for driving the compressor, the high-pressure compressor and the second synchronous machine at idle during the starting phase of the fuel cell, as well as for charging the storage battery and feeding the loads operated on the network of the storage battery after the starting phase.
In a preferred embodiment, the device is arranged in a vehicle, which has at least one electric motor which can be connected to the fuel cell for driving at least two wheels on the same axle. The device is particularly suitable for motor vehicles having at least one driving motor, in which short starting times can be achieved. Because of the starting of the motor vehicles, in particular during frequent journeys, short starting times are important in order to reduce annoying waiting times until the start of a journey.
The unit expediently contains three slip rings which are connected to the field windings of the synchronous machines and of which one is common to both field windings, which results in a saving in slip rings.
The present invention provides a method of operating a device having a fuel cell which is fed with an oxidant, in particular air, as well as by a compressor, and having a high-pressure compressor intended to supply a fuel-gas production system with air, as well as a unit having two synchronous machines which are rigidly connected to each other and of which one can be connected via an inverter to the storage battery and the second can be connected to the electrical outputs of the fuel cell. The two machine are mechanically connected at least to the high-pressure compressor and the compressor, and includes that according to the invention, in order to start the fuel cell, the first synchronous machine is energized and operated as a motor, with the second synchronous machine idling, to drive the compressor and the high-pressure compressor. The feed of fuel gas is opened, and the second synchronous machine, as a generator, is energized to a voltage below the voltage output by the fuel cell in operation. After the operating voltage of the fuel cell has been reached, the second synchronous machine has the fuel-cell voltage applied to it at this voltage located below the fuel-cell voltage and, as a motor, drives the compressor and high-pressure compressor and the first synchronous machine.
In the method according to the invention, during the starting operation or the starting phase, the first synchronous machine drives additional units which effect a rapid start. That is, brings the fuel cell in a short time into a state in which it outputs its desired voltage and can output the desired power. During the starting operation, the second synchronous machine is energized to an output voltage which is somewhat lower than the fuel-cell output voltage in rated operation. As soon as the fuel-cell voltage has been reached, it is applied, via a switch and the inverter, to the second synchronous machine, as a result of which the second synchronous machine changes virtually jolt-free from generator operation to motor operation. By means of the second synchronous machine, operating as a motor, the first synchronous machine is changed over to generator operation, in which it charges the storage battery and supplies the electrical loads connected to the storage battery network with power.
It is expedient for the first synchronous machine to have field current applied to it first and then for power to be fed into the stator winding through the inverter. This is beneficial for the production of an appropriate starting torque.
In a preferred embodiment, a water pump is connected to the high-pressure compressor in a cooling circuit of the fuel cell and, during the stator operation of the fuel cell, is changed over to bypass operation, the feed to the cooling circuit to the fuel cell being shut off. The fuel cell is not cooled in the starting phase, in which the heat produced in it is used to heat it up to operating temperature. Only under load conditions in which a high development of heat takes place is the fuel cell supplied with cooling water.
The invention is explained in more detail in the following text using an exemplary embodiment which is illustrated in a drawing and from which further details, features and advantages emerge.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.